Scanning the scans: CT and MRI

Modern technology has revolutionized the practice of medicine. Innovations and improvements arrive so quickly that doctors sometimes have new whiz-bang methods at their disposal before they know how best to use them. And no field in medicine has changed more rapidly than diagnostic imaging.

Powerful new tools allow doctors to look deep inside your body without a scalpel or stitch, enabling them to diagnose diseases with remarkable precision and to spot trouble early, when it can be treated most effectively. Indeed, CT scans and MRIs have already proven their mettle. They have become invaluable diagnostic tools in today’s medical practice.

X-Rays: Humble Beginnings

It all started in 1895, when Wilhelm Roentgen discovered x-rays. Six years later, he earned the first Nobel Prize in physics. By then, doctors were already using x-rays to diagnose problems, and the collaboration between physicists and physicians was under way.

Although they are still called roentgenograms, x-rays have improved dramatically over the past century. Ordinary x-rays are important diagnostic tools for many medical problems, from fractures to pneumonia. A stationary tube beams x-rays through the patient’s body. Tissues that are dense, such as bones, stop the x-rays from penetrating the body, while less dense tissues, such as muscles and fat, allow them to pass through to a sheet of film behind the patient. When the film is developed, the dense tissues appear white, the less dense black or various shades of gray.

New digital techniques have made x-rays sharper and clearer than ever. Digital imagery also allows x-rays to be viewed and stored electronically, eliminating bulky film, and doctors can transmit the images to physicians anywhere in the world in an instant. Digital technology, though, can’t overcome the intrinsic limitations of x-rays; they can’t produce images of tissues that are not dense. Healthy lungs, for example, appear uniformly black because they are filled with air, but if lungs fill up with fluid (congestive heart failure) or pus (pneumonia), the abnormal area looks white on the film or screen. By using radio-dense contrast materials, doctors can obtain images of tissues that otherwise allow x-rays to pass right through: Examples include angiography for blood vessels, barium swallows and enemas for the gastrointestinal tract, and intravenous pyelography (IVP) for the kidneys and urinary tract.

X-rays carry energy, and the energy can damage tissues if the dose is too high. Many of the x-ray pioneers, both physicists and physicians, paid a steep price for their discoveries, but in the process, they learned how to make x-rays safer. A 2004 study, for example, estimates that diagnostic x-rays account for less than one percent of all malignancies in the United States. Still, it’s a reminder that you should get x-rays only when you truly need them.

The CT Scan: A Leap Forward

Computed tomography (CT scanning, sometimes called computer-assisted tomography or CAT scanning) burst on the scene in the early 1970s, altering the imaging landscape forever. And rapidly evolving developments are making it better than ever. CTs use x-rays to image the body, but their relationship to ordinary x-rays is something like the link between the space shuttle and the Spirit of St. Louis. In the first-generation CT scan (the axial CT), the patient lies on a table that moves through a doughnut-shaped tube containing the x-ray generator (please see figure). The generator rotates around the patient, beaming x-rays through his body. X-rays which pass through are collected by detectors that channel the electrical signals into computers, where they are reconstructed into detailed images of the body.

In the first-generation CTs, the table moves the patient for a short distance, then stops while an image is obtained. The process is repeated until the scan is complete. But in the second-generation scans, spiral (or helical) CTs, the table moves him through the scanner without stopping while the tube rotates around him continuously in a spiral fashion (please see figure). The computer compensates for the effects of motion, constructing more detailed images than were formerly possible. Spiral CTs don’t expose the patient to any more radiation than ordinary CTs, and they are also much faster. Spiral CTs can provide remarkable detailed images of the nervous system, chest, and abdomen. In the urinary tract, for example, they have replaced the older methods of looking for kidney stones.

Even as many hospitals are proudly employing their shiny new spiral scanners, other centers are shunting them aside to install third-generation multislice (also called ultrafast or multidetector) CTs. First- and second-generation CTs use a single row of detectors to pick up the x-rays that pass through the patient; multislice scanners have up to eight rows, along with improved computer software (please see figure). As a result, they are remarkably fast, able to scan a patient’s entire chest, abdomen, and pelvis while he holds his breath for just 20 seconds to obtain much finer slices, which provides vivid images of the body in 1-mm segments. High-resolution CT images are a major advance. For example, they enable doctors to detect tiny clots in the lungs, tumors that are too small to show up with other methods, and small flecks of calcium in a coronary artery. It’s a great achievement – but now comes the hard part, learning how to apply the possibilities of physics to the practicalities of patient care.

CT scanning is a remarkable tool. And by administering contrast material to blood vessels (CT angiography) or the intestinal tract (virtual colonoscopy), doctors can expand its use. However, CTs are much more expensive than ordinary x-rays. In addition, some experts are concerned that because they are so good, they are used more often than really necessary.

Next Up: MRIs

CTs have not replaced x-rays, and MRIs won’t push CT scanners from hospitals to museums. Still, magnetic resonance imaging can offer major advantages, at least for certain patients. Instead of relying on radiation, MRIs use the body’s natural magnetic properties to produce detailed images of any part of the body. Because hydrogen is so abundant in the body, especially in water and fat, it is used as the target. Like the planet Earth, the hydrogen nucleus – just a single proton particle – rotates on an axis. Also like Earth, every hydrogen proton behaves like a bar magnet, with north and south poles.

Under normal conditions, the magnetic poles are randomly aligned and don’t generate enough energy to produce images. But when the body is placed in a strong magnetic field, the proton "magnets" line up along an axis, like so many parallel compass needles. If the protons are then hit with a short burst of precisely tuned radio waves, they will momentarily turn around. Then, in the process of returning to their original orientation, they emit a brief radio signal of their own.

To obtain an MRI, a technician places the patient in a long tube that produces an intense magnetic field and generates pulses of radio waves. The tissues’ hydrogen protons resonate, emitting radio signals that are captured by detectors and processed by a computer into detailed images. By alternating the sequence of pulses, doctors can use MRIs to obtain images of tissues anywhere in the body. And by using a contrast agent (gadolinium), doctors can further enhance the MRI.

MRIs sound scary, but since they don’t use radiation, they are safe for all of the body’s tissues. Still, they frighten some patients, who develop claustrophobia when they are confined to a narrow, noisy tube for 30 minutes or more. Sedatives can help, and new open scanners are less threatening. Patients who have metal in their bodies (such as pacemakers, inner ear implants, clips on brain aneurysms, some artificial joints, and shrapnel) cannot be given MRIs (though they can have CT scans). MRIs are also very expensive.

Which Scan?

Plain old x-rays are fine for many problems, from pneumonia and heart failure to fractures and dislocations. CT is the current champ for mobile areas of the body, including the lungs and abdomen. Because they are faster than MRIs, CTs are usually preferred for trauma patients, even when the brain is involved. Aside from trauma though, doctors generally prefer MRIs to CTs for precise images of the brain and the nervous system. MRIs also shine for diseases of the joints, muscles, and possibly for the pelvic organs.

Which scan is best for you? It depends on your problem. Your doctor can help make this decision for you depending on your particular case. But if you do your best to stay healthy, you won’t need any scans, at least until research produces still other unimagined advances in imaging!